Internet Engineering Task Force (IETF) M. Groves
Request for Comments: 6509 CESG
Category: Informational February 2012
ISSN: 2070-1721
MIKEY-SAKKE: Sakai-Kasahara Key Encryption inMultimedia Internet KEYing (MIKEY)
Abstract
This document describes the Multimedia Internet KEYing-Sakai-Kasahara
Key Encryption (MIKEY-SAKKE), a method of key exchange that uses
Identity-based Public Key Cryptography (IDPKC) to establish a shared
secret value and certificateless signatures to provide source
authentication. MIKEY-SAKKE has a number of desirable features,
including simplex transmission, scalability, low-latency call setup,
and support for secure deferred delivery.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It has been approved for publication by the Internet
Engineering Steering Group (IESG). Not all documents approved by the
IESG are a candidate for any level of Internet Standard; see Section2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6509.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
M. Groves Informational [Page 1]

RFC 6509 MIKEY-SAKKE February 20121. Introduction
Multimedia Internet KEYing (MIKEY) [RFC3830] defines a protocol
framework for key distribution and specifies key distribution methods
using pre-shared keys, RSA, and, optionally, a Diffie-Hellman Key
Exchange. Since the original specification, several alternative key
distribution methods for MIKEY have been proposed such as [RFC4650],
[RFC4738], [RFC6043], and [RFC6267].
This document describes MIKEY-SAKKE, a method for key exchange and
source authentication designed for use in IP Multimedia Subsystem
(IMS) [3GPP.33.328] Media Plane Security, but with potential for
wider applicability. This scheme makes use of a Key Management
Service (KMS) as a root of trust and distributor of key material.
The KMS provides users with assurance of the authenticity of the
peers with which they communicate. Unlike traditional key
distribution systems, MIKEY-SAKKE does not require the KMS to offer
high availability. Rather, it need only distribute new keys to its
users periodically.
MIKEY-SAKKE consists of an Identity-based Public Key Cryptography
(IDPKC) scheme based on that of Sakai and Kasahara [S-K], and a
source authentication algorithm that is tailored to use Identifiers
instead of certificates. The algorithms behind this protocol are
described in [RFC6507] and [RFC6508].
The primary motivation for the MIKEY protocol design is the low-
latency requirement of real-time communication; hence, many of the
defined exchanges finish in one-half to one roundtrip. However, some
exchanges, such as those described in [RFC6043] and [RFC6267], have
been proposed that extend the latency of the protocol with the intent
of providing additional security. MIKEY-SAKKE affords similarly
enhanced security, but requires only a single simplex transmission
(one-half roundtrip).
MIKEY-SAKKE additionally offers support for scenarios such as
forking, retargeting, deferred delivery, and pre-encoded content.
1.1. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
M. Groves Informational [Page 3]

RFC 6509 MIKEY-SAKKE February 20122. A New MIKEY Mode: MIKEY-SAKKE2.1. Outline
The proposed MIKEY mode requires a single simplex transmission. The
Initiator sends a MIKEY I_MESSAGE containing SAKKE Encapsulated Data
and a signature to the intended recipient. The Responder MUST
validate the signature. Following signature validation, the
Responder processes the Encapsulated Data according to the operations
defined in [RFC6508] to derive a Shared Secret Value (SSV). This SSV
is used as the TGK (the TEK Generation Key defined in [RFC3830]).
A verification message from the Responder (as in pre-shared key mode,
for example) is not needed, as the parties are mutually authenticated
following processing of the single I_MESSAGE. The notation used for
MIKEY messages and their payloads in Figure 1, and in the rest of
this document, is defined in [RFC3830].
Initiator Responder
I_MESSAGE =
HDR, T, RAND, [IDRi], [IDRr], [IDRkmsi], [IDRkmsr],
[CERT], {SP}, SAKKE, SIGN --->
Figure 1: MIKEY-SAKKE Unicast Mode
The Initiator wants to establish a secure media session with the
Responder. The Initiator and the Responder trust a third party, the
KMS, which provisions them with key material by a secure mechanism.
In addition to the public and secret keys corresponding to their
Identifier, the KMS MUST provision devices with its KMS Public Key
and, where [RFC6507] is used, its KMS Public Authentication Key. A
description of all key material used in MIKEY-SAKKE can be found in
Section 2.1.2. The Initiator and the Responder do not share any
credentials; instead, the Initiator is able to derive the Responder's
public Identifier.
Implementations MAY provide support for multiple KMSs. In this case,
rather than a single KMS, several different KMSs could be involved,
e.g., one for the Initiator and one for the Responder. To allow
this, each interoperating KMS MUST provide its users with the KMS
public keys for every KMS subscriber domain with which its users
communicate. It is not anticipated that large mutually communicating
groups of KMSs will be needed, as each KMS only needs to provide its
domain of devices with key material once per key period (see
Section 3.3) rather than to be active in each call.
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RFC 6509 MIKEY-SAKKE February 2012
As MIKEY-SAKKE is based on [RFC3830], the same terminology,
processing, and considerations still apply unless otherwise stated.
Following [RFC3830], messages are integrity protected and encryption
is not applied to entire messages.
2.1.1. Parameters
[RFC6508] requires each application to define the set of public
parameters to be used by implementations. The parameters in
Appendix A SHOULD be used in MIKEY-SAKKE; alternative parameters MAY
be subsequently defined; see Section 4.2.
[RFC6507] requires each application to define the hash function and
various other parameters to be used (see Section 4.1 of [RFC6507]).
For MIKEY-SAKKE, the P-256 elliptic curve and base point [FIPS186-3]
and SHA-256 [FIPS180-3] MUST be used.
2.1.2. Key Types
Users require keys for [RFC6508] and to sign messages. These keys
MUST be provided by the users' KMS. It is RECOMMENDED that
implementations support the scheme for signatures described in
[RFC6507]. Alternatively, RSA signing as defined in [RFC3830] MAY be
used.
SAKKE keys
SAKKE requires each user to have a Receiver Secret Key, created by
the KMS, and the KMS Public Key. For systems that support
multiple KMSs, each user also requires the KMS Public Key of every
KMS subscriber domain with which communication is authorized.
ECCSI keys
If the Elliptic Curve-based Certificateless Signatures for
Identity-based Encryption (ECCSI) signatures are used, each user
requires a Secret Signing Key and Public Validation Token, created
by the KMS, and the KMS Public Authentication Key. For systems
that support multiple KMSs, each user also requires the KMS Public
Authentication Key of every KMS subscriber domain with which
communication is authorized.
If instead RSA signatures are to be used, certificates and
corresponding private keys MUST be supplied.
M. Groves Informational [Page 5]

RFC 6509 MIKEY-SAKKE February 20122.2. Preparing and Processing MIKEY-SAKKE Messages
Preparation and parsing of MIKEY messages are as described in
Sections 5.2 and 5.3 of [RFC3830]. Error handling is described in
Section 5.1.2, and replay protection guidelines are in Section 5.4 of
[RFC3830]. In the following, we describe the components of
MIKEY-SAKKE messages and specify message processing and parsing rules
in addition to those in [RFC3830].
2.2.1. Components of the I_MESSAGE
MIKEY-SAKKE requires a single simplex transmission (a half roundtrip)
to establish a shared TGK. The I_MESSAGE MUST contain the MIKEY
Common Header Payload HDR defined in [RFC6043] together with the
timestamp payload in order to provide replay protection. The HDR
field contains a CSB_ID (Crypto Session Bundle ID) randomly selected
by the Initiator. The V bit in the HDR payload MUST be set to '0'
and ignored by the Responder, as a response is not expected in this
mode. The timestamp payload MUST use TS type NTP-UTC (TS type 0) or
NTP (TS type 1) as defined in Section 6.6 of [RFC3830] so that the
Responder can determine the Identifiers used by the Initiator (see
Section 3.2). It is RECOMMENDED that the time always be specified
in UTC.
The I_MESSAGE MUST be signed by the Initiator following either the
procedure to sign MIKEY messages specified in [RFC3830], or using
[RFC6507] as specified in this document. The SIGN payload contains
this signature. Thus, the I_MESSAGE is integrity and replay
protected. The ECCSI signature scheme [RFC6507] SHOULD be used. If
this signature scheme is used, then the Initiator MUST NOT include a
CERT payload. To form this signature type, the Initiator requires a
Secret Signing Key that is provided by the KMS.
Other signature types defined for use with MIKEY MAY be used. If
signature types 0 or 1 (RSA) are used, then the Initiator SHOULD
include a CERT payload; in this case, the CERT payload MAY be left
out if it is expected that the Responder is able to obtain the
certificate in some other manner. If a CERT payload is included, it
MUST correspond to the private key used to sign the I_MESSAGE.
The Initiator MUST include a RAND payload in the I_MESSAGE, as this
is used to derive session keys.
The identities of the Initiator, Responder, the Initiator's KMS (root
of trust for authentication of the Initiator), and the Responder's
KMS (root of trust for authentication of the Responder) MAY be
contained in the IDRi, IDRr, IDRkmsi, and IDRkmsr I_MESSAGEs,
respectively. The ID Payload with Role Indicator (IDR) is defined in
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RFC 6509 MIKEY-SAKKE February 2012
[RFC6043] and modified in Section 4.4. When used, this payload
provides the Identifier for any of the Initiator, the Responder, and
their respective KMSs.
The ID Role MUST be the Initiator (value 1) for the IDRi payload and
Responder (value 2) for the IDRr payload. The Initiator's ID is used
to validate signatures [RFC6507]. If included, the IDRi payload MUST
contain the URI of the Initiator incorporated in the Identifier used
to sign the I_MESSAGE (see Section 3.2). If included, the IDRr
payload MUST contain the URI of the Responder incorporated in the
Identifier that the Initiator used in SAKKE (see Section 3.2). If
included, the ID Role MUST be the Initiator's KMS (value 6) for the
IDRkmsi payload and Responder's KMS (value 7) for the IDRkmsr payload
and MUST correspond to the KMS used as root of trust for the
signature (for the IDRkmsi payload) and the KMS used as the root of
trust for the SAKKE key exchange (for the IDRkmsr payload).
It is OPTIONAL to include any IDR payloads, as in some user groups
Identifiers could be inferred by other means, e.g., through the
signaling used to establish a call. Furthermore, a closed user group
could rely on only one KMS, whose identity will be understood and
need not be included in the signaling.
The I_MESSAGE MUST contain a SAKKE payload constructed as defined in
Section 4.2.
The Initiator MAY also send security policy (SP) payload(s)
containing all the security policies that it supports. If the
Responder does not support any of the policies included, it SHOULD
reply with an error message of type "Invalid SPpar" (Error no. 10).
The Responder has the option not to send the error message in MIKEY
if a generic session establishment failure indication is deemed
appropriate and communicated via other means (see Section 4.1.2 of
[RFC4567] for additional guidance).
2.2.2. Processing the I_MESSAGE
The Responder MUST process the I_MESSAGE according to the rules
specified in Section 5.3 of [RFC3830]. The following additional
processing MUST also be applied.
* If the Responder does not support the MIKEY-SAKKE mode of
operation, or otherwise cannot correctly parse the received MIKEY
message, then it SHOULD send an error message "Unsupported message
type" (Error no. 13). Error no. 13 is not defined in [RFC3830],
and so implementations compliant with [RFC3830] MAY return an
"Unspecified error" (Error no. 12).
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RFC 6509 MIKEY-SAKKE February 2012
* The Responder MAY compare the IDi payload against his local policy
to determine whether he wishes to establish secure communications
from the Initiator. If the Responder's policy does not allow this
communication, then the Responder MAY respond with an "Auth
failure" error (Error no. 0).
* If the Responder supports MIKEY-SAKKE and has determined that it
wishes to establish secure communications with the Initiator, then
it MUST verify the signature according to the method described in
Section 5.2.2 of [RFC6507] if it is of type 2, or according to the
certificate used if a signature of type 0 or 1 is used. If the
verification of the signature fails, then an "Auth failure" error
(Error no. 0) MAY be sent to the Initiator.
* If the authentication is successful, then the Responder SHALL
process the SAKKE payload and derive the SSV according to the
method described in [RFC6508].
2.3. Forking and Retargeting
Where forking is to be supported, Receiver Secret Keys can be held by
multiple devices. To facilitate this, the Responder needs to load
his Receiver Secret Key into each of his devices that he wishes to
receive MIKEY-SAKKE communications. If forking occurs, each of these
devices can then process the SAKKE payload, and each can verify the
Identifier of the Initiator as they hold the KMS Public
Authentication Key. Therefore, the traffic keys could be derived by
any of these devices. However, this is the case for any scheme
employing simplex transmission, and it is considered that the
advantages of this type of scheme are significant for many users.
Furthermore, it is for the owner of the Identifier to determine on
which devices to allow his Receiver Secret Key to be loaded. Thus,
it is anticipated that he would have control over all devices that
hold his Receiver Secret Key. This argument also applies to
applications such as call centers, in which the security relationship
is typically between the call center and the individual calling the
center, rather than the particular operative who receives the call.
Devices holding the same Receiver Secret Key ought to each hold a
different Secret Signing Key corresponding to the same Identifier.
This is possible because the Elliptic Curve-based Certificateless
Signatures for Identity-based Encryption (ECCSI) scheme allows
multiple keys to be generated by KMS for the same Identifier.
Secure retargeted calls can only be established in the situation
where the Initiator is aware of the Identifier of the device to whom
the call is being retargeted; in this case, the Initiator ought to
initiate a new MIKEY-SAKKE session with the device to whom it has
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RFC 6509 MIKEY-SAKKE February 2012
been retargeted (if willing to do so). Retargeting an Initiator's
call to another device (with a different Identifier) is to be viewed
as insecure when the Initiator is unaware that this has occurred, as
this prevents authentication of the Responder.
2.4. Group Communications
SAKKE supports key establishment for group communications. The
Initiator needs to form an I_MESSAGE for each member in the group,
each using the same SSV. Alternatively, a bridge can be used. In
this case, the bridge forms an I_MESSAGE for each member of the
group. Any member of the group can invite new members directly by
forming an I_MESSAGE using the group SSV.
2.5. Deferred Delivery
Deferred delivery / secure voicemail is fully supported by MIKEY-
SAKKE. A deferred delivery server that supports MIKEY-SAKKE needs to
store the MIKEY-SAKKE I_MESSAGE along with the encrypted data. When
the recipient of the voicemail requests his data, the server needs to
initiate MIKEY-SAKKE using the stored I_MESSAGE. Thus, the data can
be received and decrypted only by a legitimate recipient, who can
also verify the Identifier of the sender. This requires no
additional support from the KMS, and the deferred delivery server
need not be trusted, as it is unable to read or tamper with the
messages it receives. Note that the deferred delivery server does
not need to fully implement MIKEY-SAKKE merely to store and forward
the I_MESSAGE.
The deferred delivery message needs to be collected by its recipient
before the key period in which it was sent expires (see Section 3.3
for a discussion of key periods). Alternatively, if greater
longevity of deferred delivery payloads is to be supported, the
Initiator needs to include an I_MESSAGE for each key period during
the lifetime of the deferred delivery message, each using the same
SSV. In this case, the deferred delivery server needs to forward the
I_MESSAGE corresponding to the current key period to the recipient.
3. Key Management3.1. Generating Keys from the Shared Secret Value
Once a MIKEY-SAKKE I_MESSAGE has been successfully processed by the
Responder, he will share an authenticated SSV with the Initiator.
This SSV is used as the TGK. The keys used to protect application
traffic are derived as specified in [RFC3830].
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RFC 6509 MIKEY-SAKKE February 20123.2. Identifiers
One of the primary features and advantages of Identity-Based
Encryption (IBE) is that the public keys of users are their
Identifiers, which can be constructed by their peers. This removes
the need for Public Key or Certificate servers, so that all data
transmission per session can take place directly between the peers,
and high-availability security infrastructure is not needed. In
order for the Identifiers to be constructable, they need to be
unambiguously defined. This section defines the format of
Identifiers for use in MIKEY-SAKKE.
If keys are updated regularly, a KMS is able to revoke devices. To
this end, every Identifier for use in MIKEY-SAKKE MUST contain a
timestamp value indicating the key period for which the Identifier is
valid (see Section 3.3). This document uses a year and month format
to enforce monthly changes of key material. Further Identifier
schemes MAY be defined for communities that require different key
longevity.
An Identifier for use in MIKEY-SAKKE MUST take the form of a
timestamp formatted as a US-ASCII string [ASCII] and terminated by a
null byte, followed by identifying data which relates to the identity
of the device or user, also represented by a US-ASCII string and
terminated by a null byte.
For the purposes of this document, the timestamp MUST take the form
of a year and month value, formatted according to [ISO8601], with the
format "YYYY-MM", indicating a four-digit year, followed by a hyphen
"-", followed by a two-digit month.
For the Identifier scheme defined in this document, the identifying
data MUST take the form of a constrained "tel" URI. If an
alternative URI scheme is to be used to form SAKKE Identifiers, a
subsequent RFC MUST define constraints to ensure that the URI can be
formed unambiguously. The normalization procedures described in
Section 6 of [RFC3986] MUST be used as part of the constraining rules
for the URI format. It would also be possible to define Identifier
types that used identifying data other than a URI.
The restrictions for the "tel" URI scheme [RFC3966] for use in
MIKEY-SAKKE Identifiers are as follows:
* the "tel" URI for use in MIKEY-SAKKE MUST be formed in global
notation,
* visual separators MUST NOT be included,
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RFC 6509 MIKEY-SAKKE February 2012
* the "tel" URI MUST NOT include additional parameters, and
* the "tel" URI MUST NOT include phone-context parameters.
These constraints on format are necessary so that all parties can
unambiguously form the "tel" URI.
For example, suppose a user's telephone number is +447700900123 and
the month is 2011-02, then the user's Identifier is defined as the
ASCII string:
2011-02\0tel:+447700900123\0,
where '\0' denotes the null 8-bit ASCII character 0x00.
If included in I_MESSAGE, the IDRi and IDRr payloads MUST contain the
URI used to form the Identifier. The value of the month used to form
the Identifiers MUST be equal to the month as specified by the data
in the timestamp payload.
3.3. Key Longevity and Update
Identifiers for use in MIKEY-SAKKE change regularly in order to force
users to regularly update their key material; we term the interval
for which a key is valid a "key period". This means that if a device
is compromised (and this is reported procedurally), it can continue
to communicate with other users for at most one key period. Key
periods SHOULD be indicated by the granularity of the format of the
timestamp used in the Identifier. In particular, the Identifier
scheme in this document uses monthly key periods. Implementations
MUST allow devices to hold two periods' keys simultaneously to allow
for differences in system time between the Initiator and Responder.
Where a monthly key period applies, it is RECOMMENDED that
implementations receive the new key material before the
second-to-last day of the old month, commence allowing receipt of
calls with the new key material on the second-to-last day of the old
month, and continue to allow receipt calls with the old key material
on the first and second days of the new month. Devices SHOULD cease
to receive calls with key material corresponding to the previous
month on the third day of the month; this is to allow compromised
devices to be keyed out of the communicating user group.
KMSs MAY update their KMS Master Secret Keys and KMS Master Secret
Authentication Keys. If such an update is not deemed necessary, then
the corresponding KMS Public Keys and KMS Public Authentication Keys
will be fixed. If KMS keys are to be updated, then this update MUST
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RFC 6509 MIKEY-SAKKE February 2012
occur at the change of a key period, and new KMS Public Key(s) and
KMS Public Authentication Key(s) MUST be provided to all users with
their user key material.
It is NOT RECOMMENDED for KMSs to distribute multiple key periods'
keys simultaneously, as this prevents the periodic change of keys
from excluding compromised devices.
3.4. Key Delivery
This document does not seek to restrict the mechanisms by which the
necessary key material might be obtained from the KMS. The
mechanisms of [RFC5408] are not suitable for this application, as the
MIKEY-SAKKE protocol does not require public parameters to be
obtained from a server: these are fixed for all users in order to
facilitate interoperability and simplify implementation.
The delivery mechanism used MUST provide confidentiality to all
secret keys, integrity protection to all keys, and mutual
authentication of the device and the KMS.
4. Payload Encoding
This section describes the new SAKKE payload and also the payloads
for which changes have been made compared to [RFC3830]. A detailed
description of MIKEY payloads is provided in [RFC3830].
4.1. Common Header Payload (HDR)
An additional value is added to the data type and next payload
fields.
* Data type (8 bits): describes the type of message.
Data type | Value | Comment
-----------------------------------------------
SAKKE msg | 26 | Initiator's SAKKE message
Table 1: Data type (additions)
* Next payload (8 bits): identifies the payload that is added after
this payload.
Next payload | Value | Section
-------------------------------
SAKKE | 26 | 4.2
Table 2: Next payload (additions)
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RFC 6509 MIKEY-SAKKE February 20124.3. SIGN Payload
To enable use of the ECCSI signature algorithm, which has efficiency
benefits for use with Identity-based encryption, we define an
additional signature type.
* S type (4 bits): indicates the signature algorithm applied by the
Signer.
S type | Value | Comments
-----------------------------------
ECCSI | 2 | ECCSI signature
Table 6: S type (additions)
4.4. IDR Payload
The IDR payload was defined in [RFC6043], but its definition only
provided the facility to identify one KMS per exchange. Since it is
possible that different KMSs could be used by the Initiator and
Responder, this payload is extended to define an ID Role for the KMS
of the Initiator and the KMS of the Responder.
* ID Role (8 bits): specifies the sort of identity.
ID Role | Value
---------------------------------
Initiator's KMS (IDRkmsi) | 6
Responder's KMS (IDRkmsr) | 7
Table 7: ID Role (additions)
5. Applicability of MIKEY-SAKKE Mode
MIKEY-SAKKE is suitable for use in a range of applications in which
secure communications under a clear trust model are needed. In
particular, the KMS need not provide high availability, as it is only
necessary to provide a periodic refresh of key material. Devices are
provided with a high level of authentication, as the KMS acts as a
root of trust for both key exchange and signatures.
6. Security Considerations
Unless explicitly stated, the security properties of the MIKEY
protocol as described in [RFC3830] apply to MIKEY-SAKKE as well. In
addition, MIKEY-SAKKE inherits some properties of Identity-based
cryptography. For instance, by concatenating the "date" with the URI
to form the Identifier, the need for any key revocation mechanisms is
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RFC 6509 MIKEY-SAKKE February 2012
virtually eliminated. It is NOT RECOMMENDED for KMSs to distribute
multiple months' keys simultaneously in an IBE system, as this
prevents the monthly change of keys from excluding compromised
devices.
The solution proposed provides protection suitable for high-security
user groups, but is scalable enough that it could be used for large
numbers of users. Traffic keys cannot be derived by any
infrastructure component other than the KMS.
The effective security of the public parameters defined in this
document is 112 bits, as this is the security offered by the prime p
of size 1024 bits used in SAKKE (see Section 7 of [RFC6508]). For
similar parameter sizes, MIKEY-SAKKE provides equivalent levels of
effective security to other schemes of this type (such as [RFC6267]).
For reasons of efficiency and security, it is RECOMMENDED to use a
mode of AES-128 [AES] in the traffic application to which MIKEY-SAKKE
supplies key material, but users SHOULD be aware that 112 bits of
security are offered by the defined public parameters. Following
[SP800-57], this choice of security strength is appropriate for use
to protect data until 2030.
User identities cannot be spoofed, since the Public Authentication
Token is tied to the Identifier of the sender by the KMS. In
particular, the Initiator is provided with assurance that nobody
other than a holder of the legitimate Receiver Secret Key can process
the SAKKE Encapsulated Data, and the signature binds the holder of
the Initiator's Secret Signing Key to the I_MESSAGE. Since these
keys are provided via a secure channel by the KMS, mutual
authentication is provided. This mechanism protects against both
passive and active attacks.
If there were a requirement that a caller remain anonymous from any
called parties, then it would be possible to remove the signature
from the protocol. A called user could then decide, according to
local policy, whether to accept such a secure session.
6.1. Forking
Where forking is used, the view is taken that it is not necessary for
each device to have a separate Receiver Secret Key. Rather, where a
user wishes his calls to be forked between his devices, he loads the
same Receiver Secret Key onto each of them. This does not compromise
his security as he controls each of the devices, and is consistent
with the Initiator's expectation that he is authenticated to the
owner of the Identifier he selected when initiating the call.
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RFC 6509 MIKEY-SAKKE February 20126.2. Retargeting
Since the Initiator is made aware by the forwarding server of the
change to the Identifier of the Responder, he creates an I_MESSAGE
that can only be processed by this legitimate Responder. The
Initiator MAY also choose to discontinue the session after checking
his local policy.
6.3. Group Calls
Any device that possesses an SSV can potentially provide it securely
to any other device using SAKKE. Thus, group calls can either be
established by an Initiator, or can be extended to further Responders
by any party to whom the original Initiator has sent an I_MESSAGE.
The Initiator in this context MAY be a conference bridge. If a mode
of operation in which a bridge has no knowledge of the SSV is needed,
the role of the MIKEY-SAKKE Initiator MUST be carried out by one or
more of the communicating parties, not by the bridge.
Where multi-way communications (rather than broadcast) are needed,
the application using the supplied key material MUST ensure that a
suitable Initialization Vector (IV) scheme is used in order to
prevent cryptovariable re-use.
6.4. Deferred Delivery
Secure deferred delivery is supported in a manner such that no trust
is placed on the deferred delivery server. This is a significant
advantage, as it removes the need for secure infrastructure
components beyond the KMS.
7. IANA Considerations
This document defines new values for the namespaces Data Type, Next
Payload, and S type defined in [RFC3830], and for the ID Role
namespace defined in [RFC6043]. The following IANA assignments have
been added to the MIKEY Payload registry:
* 26 - Data type (see Table 1)
* 26 - Next payload (see Table 2)
* 2 - S type (see Table 6)
* ID Role (see Table 7)
* 6 - Initiator's KMS (IDRkmsi)
* 7 - Responder's KMS (IDRkmsr)
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RFC 6509 MIKEY-SAKKE February 2012
The SAKKE payload defined in Section 4.2 defines two fields for which
IANA has created and now maintains namespaces in the MIKEY Payload
registry. These two fields are the 8-bit SAKKE Params field, and the
8-bit ID Scheme field. IANA has recorded the pre-defined values
defined in Section 4.2 for each of the two name spaces. Values in
the range 1-239 SHOULD be approved by the process of Specification
Required, values in the range 240-254 are for Private Use, and the
values 0 and 255 are Reserved according to [RFC5226].
Initial values for the SAKKE Params registry are given below.
Assignments consist of a SAKKE parameters name and its associated
value.
Value SAKKE params Definition
----- ------------ ----------
0 Reserved
1 Parameter Set 1 See Appendix A
2-239 Unassigned
240-254 Private Use
255 Reserved
Initial values for the ID scheme registry are given below.
Assignments consist of a name of an identifier scheme name and its
associated value.
Value ID Scheme Definition
----- ------------ ----------
0 Reserved
1 tel URI with monthly keys See Section 3.2
2-239 Unassigned
240-254 Private Use
255 Reserved
8. References8.1. Normative References
[AES] NIST, "Advanced Encryption Standard (AES)", FIPS PUB 197,
November 2001, http://www.itl.nist.gov/fipspubs/by-num.htm.
[ASCII] American National Standards Institute, "Coded Character
Sets - 7-Bit American National Standard Code for
Information Interchange (7-Bit ASCII)", ANSI X3.4, 1986.
[FIPS180-3] Federal Information Processing Standards Publication
(FIPS PUB) 180-3, "Secure Hash Standard (SHS)",
October 2008.
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